How to Size an FRP/GRP Water Tank for Your Project — The Complete Calculation Guide
Oversizing costs money. Undersizing creates water stress. Getting the tank capacity right from the start means your project runs on time and your system performs as designed. This guide walks through the sizing calculation for four common application types — with worked examples you can apply directly.
THE CORE FORMULA
The Basic Sizing Formula — Start Here
|
FORMULA |
Tank Capacity = Daily Demand × Reserve Period (days) + System Dead Volume |
▸ Daily Demand = the volume of water consumed or processed in a 24-hour period
▸ Reserve Period = the number of days of supply the tank must hold independent of supply (typically 1–5 days)
▸ System Dead Volume = the volume that stays in the tank below the outlet (typically 5–10% of total — add this as a safety margin)
WORKED EXAMPLES
Four Worked Examples — From Residential to Industrial
Example 1 — Hospital (200 bed capacity)
| Specification | Detail |
|---|---|
| Daily water demand | 200 beds × 200 litres/bed/day = 40,000 litres/day (WHO standard for acute care hospitals) |
| Reserve period | 3 days (hospital critical facility — cannot run dry) |
| Calculation | 40,000 × 3 = 120,000 litres |
| System dead volume (8%) | + 9,600 litres |
| Recommended minimum capacity | 130,000 litres — specify a 150,000L tank for margin |
| Suggested product | GRP sectional panel tank, 150,000L, food-grade isophthalic resin |
Example 2 — Office Building (300 occupants)
| Specification | Detail |
|---|---|
| Daily water demand | 300 occupants × 45 litres/person/day (office standard) = 13,500 litres/day |
| Reserve period | 2 days |
| Calculation | 13,500 × 2 = 27,000 litres |
| System dead volume (8%) | + 2,160 litres |
| Recommended minimum capacity | 30,000 litres — specify a 30,000L or 35,000L tank |
| Suggested product | GRP sectional panel tank, 30,000L, standard polyester resin |
Example 3 — Industrial Process Water (food and beverage plant)
| Specification | Detail |
|---|---|
| Process water demand | Estimated from production batch volume × batches per day — example: 5,000L/batch × 6 batches = 30,000L/day |
| Reserve period | 1.5 days (production continuity, not life safety) |
| Calculation | 30,000 × 1.5 = 45,000 litres |
| System dead volume (10%) | + 4,500 litres |
| Recommended minimum capacity | 50,000 litres — specify a 50,000L tank |
| Suggested product | GRP sectional or cylindrical tank, 50,000L, food-grade isophthalic resin, NSF-61 |
Example 4 — Fire Fighting Tank (office + warehouse complex)
|
Specification |
Detail |
|---|---|
|
System type |
Sprinkler system covering 5,000m² mixed-occupancy building |
|
Required flow rate |
1,500 litres/minute (confirm with fire engineer to BS EN 12845 or NFPA 13) |
|
Required duration |
60 minutes |
|
Calculation |
1,500 × 60 = 90,000 litres |
|
Safety margin (15%) |
+ 13,500 litres |
|
Recommended minimum capacity |
103,500 litres — specify a 110,000L fire tank |
|
Suggested product |
GRP fire fighting tank, 110,000L, with dedicated fire outlet and ballcock |
|
|
FIRE SIZING DISCLAIMER: Fire tank sizing must always be confirmed by a qualified fire engineer against the applicable standard (NFPA 22, BS EN 12845, or local fire code). The example above is illustrative only. Do not use it as a design basis without fire engineering review. |
DEMAND REFERENCE TABLE
Water Demand Reference — Common Nigerian Applications
|
Application |
Typical Daily Demand per Unit |
|---|---|
|
Residential (per person) |
80–150 litres/person/day |
|
Office building (per occupant) |
40–50 litres/person/day |
|
Hospital (per bed, acute care) |
200–400 litres/bed/day |
|
School (per student) |
25–50 litres/student/day |
|
Hotel (per room) |
200–350 litres/room/day |
|
Restaurant (per cover per service) |
30–50 litres/cover |
|
Light industrial (per employee) |
50–80 litres/employee/day |
|
Food and beverage (process water) |
Calculated from process — consult production engineer |
FAQ
FRP Tank Sizing — Common Questions
My site has intermittent mains supply — how does that affect sizing?
A: For sites with unreliable mains supply, the reserve period should be extended to cover the typical gap between supply interruptions — commonly 3–7 days in areas with irregular supply. This increases the required tank capacity significantly. Contact us to discuss your supply profile and we will help you size correctly.
I'm not sure what my daily demand is — can you help?
A: Yes — contact Karoch Engineering with your building type, number of occupants or production volume, and any existing metering data. We will help you estimate demand and recommend a tank capacity accordingly.
Can I increase the tank capacity later without replacing the whole tank?
A: Yes — GRP sectional panel tanks can be expanded by adding panels. This is one of their key advantages. However, the concrete base must be sized for the expanded footprint from the start. Tell us your expansion plans when we quote the initial tank.
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